Directive radio aerial systems



FebQll, 1958 s. coRNBLr-:ET 2,823,380

-DIRECTIVE RADIO AERIAL SYSTEMS Filed July 20, 1954 2 SheQbS-Sheec 1 NysNToA Feb. 11, 1958 Filed July 20, 1954 s. CORNBLEET 2,823,380

DIRECTIVE RADIO AERIAL SYSTEMS 2 sheets-shew 2 @RRRRR FIG. 4.

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` TVE-mede] l United States PatentI DIRECTIVE RADIO AERIAL SYSTEMS Sidney Cornbleet, Edgware, England, assignor to The` The present invention relates to directive radio aerial systems of the kind known as reverse feed aerial systems. In reverse feed directive radio aerial systems, there are provided-a parabolic orv other reflector, a waveguide or coaxial cable feeder projecting through the reflector from behind, and means for directing radio waves from (or to) the end of the feeder on to (or from) the reflector and thence into (or from) space. Reverse feed aerial systems may be employed either as simple transmitting or receiving aerials,v or in duplex operation as common transmitting-receiving aerials. Itvwill be understood that where the operation of aerials is described in this specification with reference to one particular application, for example transmitting, they may equally well be used for either of the other applications.

Usually in reverse feeddirective radio aerial systems wherethe reflector is parabolic, the feeder is mainly co-V axial with the reflector, so that the direction of waves travelling along the feeder has substantially to be reversed atthe end of the feeder in' order to direct them on to the reflector. One known method of achieving this is to pro- -vide a subsidiary reflector placed opposite the termina- :tion of the feeder, and arranged to reflect waves radiated from the feeder termination on to the main reflector. The :subsidiary reflector may be simply a plane plate,` or it :may be shaped, for example it may be hyperbolic. One` is a waveguide. In thisV the end of the waveguide is ta'`v pered and projects into a hollow metal box. The box has .apertures in its walls which are directed towards the reflector, and from which waves are radiated on to the reflector. The apertures may be sealed with a dielectric material, for example mica, in order to seal the waveguide from the atmosphere. -A matching slug, which acts in effect as a variable reactance, is provided in the wall of the box opposite the end of the waveguide. This arrangement suffers from the disadvantage that the radio frequency power which may be passed through it is limited, particularly by the narrow depth of the box and by the taper at the end of the waveguide which is necessary in order to allow the apertures to be close enough for their phase centres to be practically coincident. -An additional and in some cases a more important limitation is` that it may only pass a narrow band of frequencies.`

2,823,380 `Patented Febrlyl, 19587' Other forms of reverse feed directive radio aerial sys-i tems are described in the specifications of copending Brit-A ish patent applications No. 22,757/ 5 1 and 24,287/ 5 l (Serial No. 706,725). These systems have improved'.v means for directing Velectromagnetic waves from (or to)' the e'nd of the feeder `on to (or from) the reflector, in a system in whichv atleast the end part of the feeder is a length of rectangular cross-section waveguide. It is an object `of the present invention to provide a further reverse feed directive radio aerial system which is similar in design and performance to those described in the afore- Asaid patent applications, but which is more convenient for manufacture on a production scale.

According to the present invention, in a reverse feed directive radio aerial system, in which at least the end section of the feeder is a length of rectangular cross-section waveguide, a pair of, resonant coupling slots are provided opposite to one another one in each of the longer dimension walls near the end of the feeder waveguide, each slot opening into a corresponding one of two equal short lengths of rectangular cross-section waveguide lying one on either side of the feeder waveguide with their axes aligned substantially parallel to those of the feeder wave-, guide, the two short lengths of waveguide each terminat. ing at their ends nearer the reflector at apertures directed towards the reflector and at their other yends at shortcircuited terminations, and the end of the feeder wave# guide also beingr terminated by a short-circuited termina-1 tion, all three said terminations and the slots being positioned to give optimum operation. v

. The system may include effectively a metal feeder reversal box mounted on the end of the feeder waveguide, thefeeder waveguide projecting into one face of the box,l the walls of the said twoshort lengths of waveguide being formed at least in part by the inner surfaces of the box and the outer longer dimension surfaces of the feeder waveguide, the apparatus lying one on either side of the. feeder `waveguide in that face of the box into which it projects, and the resonant coupling slots being situated in the vwalls of the feeder waveguide at a point along itsl length lying'within the box.

To assist in obtaining the desired electrical character`` y istics for the system, there may be provided a step V.on the tion.

inner surfaces of one at least of the longer dimension Walls of each of the two short lengths of waveguide, the steps being in a direction to increase the 'shorter dimen sion of the two short lengths of waveguide when passing;v

` be described by way of example with reference to the accompanying drawings in which,

Figure l shows a general view of the system, j Figure 2 shows a perspective view of the end of the feeder from the direction of the reector with one side;

l of the feed reversal box removed,

A, axis,

Figure '3 shows a cross-section of the end part-of the` feeder in the plane parallel to the shorter dimension of the feed cross-section and passing through its longitudinal' Figure 4 shows a section at the plane `IkV-s-,IV in ,Figure 3, and

Figure shows a perspective view of one half of the feed reversal box alone.

Referring first to Figure 1 of the accompanying drawings it will be seen that the system includes a length of rectangular waveguide feeder 1 which passes through the vertex of a paraboloidal reflector 2 from behind, its longitudinal axis lying `along the axis of the reflectorV 2. The end 3 of the feeder behind the reflector 2 is coupled in known manner at any desireddistance .from the reector 2 to an associated transmitter or receiver, or toa common transmit-receive unit if the aerial system is used in a duplex system and has to be coupled to both a 4transmitter and a receiver. The extent of the feeder 1 beyond the end 3 may be small if the aerial system is mounted close to the associated equipment, or .may be considerable and extend for exam-ple the length of a mast at the top of which the aerial system is mounted and at the foot of which the associated equipment is situated. The part of the feeder 1 behind the reflector 2 may include one or more rotatable 'coupling joints along its lengthso that the aerial system may be rotated about one or more axes.

The other end of the waveguide feeder 1 terminates at a feed reversal box 4, a pair of apertures 5 in which, lying one on either side of the feeder 1, radiate or receive energy on to or from the reflector 2. The feed reversal box 4 is described in detail below with reference to Figures 2-5 of the accompanying drawings. The aerial system being described is for operation at a band of frequencies centred about the frequency corresponding to a wavelength in space of 3.4 cms. and all dimensions given are for a system for operation at that frequency and will have to be altered correspondingly for other frequencies of operation. The diameter of the reflector 2 at the aperture is 30" and the distance of the apertures 5 from the vertex of the reector 2 is 10.5", the focal length ofthe reector 2.

Referring now also to Figures 2, 3 and 4 of the accompanying drawings, it will be seen that the feeder 1 is a length of rectangular cross-section waveguide, the shorter dimension of which is parallel to the plane of Figure 3 and the longer dimension of which is parallel to the plane of Figure 4. The waveguide used in the actual system being described is the standard waveguide No. 16, which has external dimensions of l" x 0.5" the wall thickness being 0.05. The end of the feeder 1 is terminated by a short circuit, a closely fitting metal block being secured in its end. Two rectangular resonant coupling slots 11 are cut opposite one another one in each of the longer dimension walls of the feeder 1 near its end. These slots 11 are 0.205" long and extend the whole distance between the inner surfaces of the shorter dimension walls of the feeder 1. The outer end of the block 10 is coplanar with the end of the walls of the feeder 1, its-length being determined so that the inner end lies that distance beyond the slots -11 which gives optimum operation.

Two H-shaped metallic pieces 12 are secured one on the outside of each of the longer dimension faces of the feeder 1, the side arms of the H-shaped pieces lying parallel to the longitudinal axis of the feeder 1. The pieces 12 are positioned along the length of the feeder 1, so that those edges v13 of thecross arms, which lie further from the reflector 2, are aligned with the edges of the slots 11 which lie nearer to the reflector 2. At the same time the length of the cross arms is equal .to the longer dimension of the waveguide measured internally so that the inner edges of the side arms lie along the shorter sides ofthe slots 11. The ends of the pieces 12 remote from the reflector 2 are coplanarwith the ends of the Afeeder A1, whilsttheir width is such-that the outer edges of the side arms overlap theedges ofthe .surfaces ofthe feeder 1 towhich they are secured. l

The feed reversal box 4, which', in Figure 2, is shown with one side removed, is constructed of two equal parts which are joined tggsther at halved igints .14 iu the (only one of which is shown) which appear vertical in Figure 2. Figure 5 of the accompanying drawings, to which reference should now be made, shows a perspective view of one half of the feed reversal box 4. Each half of the box 4 comprises a rectangular channel element comprising a base wall 15 and two side walls 16 and contains a pair of ramps 17, which have level faces 1S parallel to the base walls 15 at the back as they are seen in Figure 5. A channel 19 is left between the ramps 17, the width of the channel being equal to the distance between the outer faces of the H-shaped pieces 12 when secured to the feeder 1. When the two halves of the box 4-,are `assembled together, the inner surfaces of the base walls 15 are separated by a distance equal vto the width ofthe .H-shaped pieces 12, so that on aesembly of the box 4 round the feeder 1 and the pieces l12, the longer edges 0f the. H-shaned pieces 12 lie against the base walls 15 of the box 4, whilst the outer faces of the pieces 12 lie against the inner faces of the ramps 17. In addition the two halves of the box 4 are positioned so that the ends of the side walls 16 are coplanar with the `endsof the H-shaped pieces 12.

Rectangular metal blocks 20 (see particularly Figure 2) vare ttedclosely into the ends of the two rectangular channels which are formed on either side of the feeder 1 between the side walls 16 of the box 4, the faces 18 ofthe ramps 17, and the outer surfaces of the longer dimension walls of the feeder 1. The inner ends of the blocks 20 Aoverlap the slots 11 slightly, their position being determined for optimum operation. In addition dielectric plugs 21 -are pushed into the apertures 5 which lieone on.either side of the feeder 1. Metal blocks 23 (Figure 2) till the cavities in the feed reversal box 4, lying between the shorter dimension walls of the feeder 1, the inner faces of the H`shaped pieces 12 and the base walls 15.

Thefstructure is secured together by soldering or brazing'the various parts in position, with the exception of the dielectric plugs 21.

It will be seen that the structure formed by the end of the feeder 1, the H-shaped pieces 12, the feed reversal box 4, and the blocks 10 and 20 provides a pair of short lengths of waveguide lying one on either side of the feeder 1, each of which is coupled to the feeder 1 by means of one of the resonant coupling slots 11. At the ends remote from the reflector 2 the said pair of shortlengths of waveguide arevshort-,circuite'd by the blocks 20, whilst the ends nearer the reector 2 terminate at the apertures 5 directed towards the reflectorZ. The longer dimension of -the'short lengths of waveguide flares towards the apertures 5, by virtue of the sloping surfaces of the ramps 17, whilst steps occur in the Vshorter dimension at the edges22 of the cross arms of the H-shaped pieces 1 2. The various` dimensions Aare vall determined experimentally to give optimum operation, i.,.e. so thatl electromagnetic waves are coupled from the feeder 1 into space, or vice versa, with the vmaximum eficiency over the required frequency rband. In operation asa transmitting system, electromagnetic waves mainly lin the TEN mode are excited in the feeder y1, lthe power being coupled equally into ,the two shortlengths of waveguide through the slots v 11..o n reachir1gthe feed reversal. box 4, and being radiated in phase from the apertures-5 on to the reector v2. yAsimilar process takesplace in the reverse direction onthe reception of electromagnetic waves from space on to Vthe reflector '2, the waves being reected on :tothe apertures v5 and then coupled in phase into5 the feederl.

The vwidth4 )fthegfeeder 1 is such that the apertures SaretOQfarapartZto be regarded as even approximately coincident as far 7as thefreflector 2 is concerned and unlessfsteps;aretakento lthe contraryfthe phase centres 1n thedfll-plak y andHfplane- Wouldnot be coincident in the feeder 1. lt is for this reason that the dielectric plugs 21vare provided, and the longer dimension of the two.. short lengths of waveguide is ared. The klength of the l time the plugs 21 provide some adjustment of the *E4 plane phase centre towards the Heplane phase centre. The final adjustment of the position of the H-plae phase centre is effected by the aring ofY ythey longer dimension of the two short lengths of waveguide as previously described. v l

It is usually necessary, if the V. S. W. R. in the feeder 1 is to be maintained at a satisfactory figure, to fit a vertex plate to the reflector 2, that is a plane circular plate cutting off the vertex of the refiector 2. The vertex plate is arranged so that the phases of the waves reliected back into the apertures 5 from the various parts of the reflector 2 and from the vertex plate are such that there is cancellation at the apertures 5 and the amount of reected power passed back into the feeder 1 is therefore slight. With the 30 diameter parabolic reector 2 as described, a vertex plate of 2.125" radius is found suitable. Whilst improving the matching of the system, the vertex plate does however adversely affect the radiation pattern of the system.

Whilst a particular construction has been described above, it will be appreciated that similar systems may be assembled in other ways.

In particular, as an alternative to the construction described above, it may be desired to employ dielectric filled waveguide, in which case the end of the feeder 1 and the feed reversal box 4 might be constructed by suitably shaping or moulding the dielectric, and depositing the metal walls on the surface of the dielectric by some electrical process.

As stated previously the system described above was designed for operation in the band of frequencies centred about the frequency corresponding to a wavelength in space of 3.2 cms., i. e. approximately 8600-9700 mc./s. The important dimensions in this case in a system having a V. S. W. R. greater than 0.9 over the band were as follows:

nearer end of the slots 11 0.089". Thickness of H-shaped pieces 12 0.064". Length of H-shaped pieces 12 1.062. Width of H-shaped pieces 12 1.400". Width of cross piece of H-shaped pieces 12-- 0.357.

Distance from edges 22 of H-shaped pieces 12 to the nearer end of the side arms Internal distance between side walls 16 of the box 4 1.368. Width of ramps 17 0.184". Length of side walls 16 of the box 4 1.062". Length of faces 18 on the ramps 17 .35". Angle of slant of the ramps 17 18 30'.

Separation of the faces 18 of the ramps 17 in the two halves of box 4 when assembled- 0.90.

Length of blocks 20 0.19.

Length of dielectric plugs 21 0.428".

Distance dielectric plugs 21 protrude from apertures 5 0.138".

This system when tested was found to have the following characteristics:

The radiation pattern from the apertures 5 alone is such 6 that the powerfalls to half of thatradiated back in -the direction of the axis of the feeder 1 in directions inclined at an angle Yof approximately 457in all planes. The illumination at the edge of thereector 2 lis l2 decibels below ythat at the vertex. y

In combination with the reflector v2, theradiation pattern is such that the power falls to half of that radiated along the axis of the reector at an angle of approximately 3 in all planes. In the H-plane, that is the plane of the longer dimension of the feeder 1, the maximum side lobe power is 25 decibels below the maximum power of the main lobe, whilstrin the E-pla'ne it 'is,30 decibels* feeder 1 and inside the box 4 reduced to approximately V2 atmospheric pressure, the system was found to pass peak powers up to 200 K-watts without breakdown.

I claim:

1. A reverse feed arrangement for a reverse feed directive radio aerial system, comprising a feeder waveguide of rectangular cross-section having a pair of resonant coupling slots opposite to one another in each of the longer dimension walls of the feeder waveguide, means to provide a shortcircuited termination to the waveguide at one end thereof, a pair of short lengths of rectangular crosssection waveguide which lie substantially parallel to the feeder waveguide one on either side of the feeder waveguide and into which the said coupling slots open, and means to provide a short-circuited termination to each of the two short lengths of waveguide at the ends thereof nearest to the end of the feeder waveguide that has the short-circuited termination, the other end of each of the two short lengths of waveguide being terminated in an aperture.

2. In a reverse feed directive radio aerial system, a reverse feed arrangement comprising a feeder waveguide, of rectangular cross-section that has a short-circuited ter-` mination at one end, a pair of short lengths of rectan-.. gular cross-section waveguide which lie one on either side,v lof the feeder waveguide with their longitudinal axes sub-t stantially parallel to that of the feeder waveguide andi; which each has a short-circuited termination to the end thereof closest to the said terminated end of the feederV waveguide and the other end terminated in an aperture and means to couple the feeder waveguide to each of the two short lengths of waveguide at points spaced a shortv distance away from the terminated ends of the feederwaveguide and of the short lengths of waveguide.

3. A reverse feed arrangement according to claim 2f wherein each of the said short lengths of waveguide ares;

out towards the aperture in a plane that is parallel to the; longer dimension walls of the feeder waveguide.

4. A reverse feed arrangement according to claim 3: wherein two members of solid dielectric materials are sea cured in the two short lengths of waveguide respectively so as each to project through the aperture at one end of the associated short length of waveguide.

5. In a reverse feed directive radio aerial system, a reverse feed arrangement comprising a box which has one side open and which presents on opposite sides of the box a pair of internal planar surfaces that are parallel to one another and lie perpendicular to the plane of the open side of the box, two pairs of further internal surfaces which lie between the said planar surfaces so that each pair of further surfaces is adjacent to one of the planar surfaces, the two surfaces of each pair of further surfaces having first planar parts which are parallel to one another and which are perpendicular to the plane of the open side of the box and to the said planar surfaces and second planar parts which are perpendicular to the said planar surfaces and which are out from the first parts to the open side of the box, a feeder waveguide of rectangular cross-section that projects through the open side of the. box and has a pair of resonant coupling slots, one in each,

of the longer dimension walls of the feeder waveguide, between the feeder waveguide and two shortlerigths of waveguide, one on either side of the feeder` waveguide, that are each formed by one of the said planar surfaces of the b ox, one pair of the said further surfaces of the` box and a longer dimension Wall ofthe feeder waveguide, and two members of solid dielectric material secured in the said two short lengths of waveguide respectively so as each to project through the open side of the box.

6. A reverse feed arrangement according to claim 5 wherein each of the longer dimension walls of the feeder Y 8 waveguide has a step in it so that the two short lengths of waveguide are each wider at the end thereof close to the open side of the box than at the other end.

References Cited in the file of this patent UNITED STATES PATENTS 2,422,184 lCutler A Y. .k June 17, 1947 2,566,900 McArthur Y Sept. 4, 1951 2,778,016 Chu Jan. 15, 1957 FOREIGN PATENTS 708,614 Great Britain May 5, 1954 

